Here we can see that Kushina was the one that taught Minato many seals !!
Here we find out that only a non-jinchuuriki can use Shiki Fūjin
But kushina was no longer a Jinchuuriki she lost the nine tails so she could use the Shiki Fūjin ( being a uzumaki and teaching Minato many seals we can assume that Kushina knows Shiki Fūjin )
So why didnt Kushina used the seal... and minato would have lived !! ( Kushina has some chakra left because Minato use her last chakra to seal in naruto so she could see naruto again )
My theory:--> A former Jinchuuriki or a Jinchuuriki can have an effect on the Death God perhaps it can influence the Death God
I belive Kishi made that statement with Minato for a purpose and we will see the Shiki Fūjin seal in the manga again and this time something big would happen !!